The present invention relates generally to an optical recording medium, an optical recording and/or reproducing method and an optical recording and/or reproducing system. More specifically, the invention relates to an optical recording medium which has a phase change type optical recording layer irradiated with light beams for reproducing and recording/reproducing information and which has a greatly improved recording density and an extended operating wavelength range by utilizing a superresolution function, and an optical recording and/or reproducing method and optical recording and/or reproducing system which is capable of providing a greatly improved recording density and an extended operating wavelength range by utilizing a superresolution function.
Optical recording media, which are irradiated with light beams for reproducing or recording/reproducing information, have been put to practical use, for various data files, such as voice data, image data and computer data, as recording devices having large capacity, rapid accessibility and medium portability, and the development thereof is expected in future. As an example of an optical recording medium, an optical disk will be described below.
As measures to enhance the density of optical disks, there are various approaches, such as the shortening of the wavelength of gas laser for an original disk cutting, the shortening of the wavelength of a semiconductor laser serving as an operating light source, the increase of the numerical aperture of an objective lens and the decrease of the thickness of an optical disk substrate. Moreover, in the case of recordable optical disks, there are various approaches, such as the mark length record and the land group record.
As effective density enhancing techniques other than these approaches, the “superresolution function” has been proposed and studied. This is a function which is obtained in a “superresolution film”. The “superresolution film” means a film having a characteristic that an optical response varies non-linearly in accordance with the intensity of irradiated light.
That is, generally used laser beams have a light-intensity distribution which is like a Gaussian distribution. The superresolution film has different response characteristics to the central portion (high intensity portion) and peripheral portion (low intensity portion) of such a light beam. By such a spatial difference in optical response, an optical mask or aperture is formed in the central or peripheral portion of an incident light spot to reduce an effective spot size, so that a high-density recording and/or reproducing can be carried out.
At first, such a superresolutionfunction was proposed as a technique which is special to optical magnetic disks. That is, optical magnetic disks use a medium wherein a magnetic film having the superresolution function is switched-connected or magnetostatic-connected to a recording layer or a reproducing layer. Then, during irradiation with regenerative light, the temperature of the film is raised to utilize the exchange force or magnetostatic force between layers, so that an optical mask or optical aperture to a part of a regenerative spot is formed in a superresolution film.
Thereafter, a proposal for using a film material, which has a non-linearly varying optical response, without using magnetic functions, was made with respect to a read only memory disk. It was found that this proposal was applicable to all types of optical disks, such as optical recording ROM media, optical recording phase-change media and dye type recordable media, in addition to optical magnetic recording media.
Such superresolution reproducing methods and superresolution reproducing films capable of being applied to various optical disks are divided broadly into a heat mode system and a photon mode system. As disclosed in, e.g., ISOM '98-Technical Digiest (P126), the former system is designed to irradiate a superresolution film with regenerative light to heat the superresolution to cause a phase transition, such as melting, in the superresolution film to change the transmittance thereof. In this system, the response time up to the formation of an optical aperture or mask is relatively long. As disclosed in, e.g., ISOM '98-Technical Digiest (p128), the latter photon mode system is designed to irradiate a superresolutionfilm with generative light to cause the electron transition in the superresolution film to change the light transmittance by the absorption saturated phenomenon. The photon mode system is characterized in that the response time up to the formation of an optical aperture or mask is relatively short.
However, conventional optical recording media having superresolution films are limited to those having a monolayer superresolution film. However, in the case of the monolayer, there is a problem in that it is difficult to reduce the size of the optical aperture and to ensure a practical operating wavelength margin.